A PetroChina natural gas station is seen in Changzhou, Jiangsu province, August 2012. [Zhen Huai/Asianewsphoto]

There are lots of ways to climb the ranks of the world's largest companies.

Apple Inc surged to the top by making must-have gadgets with big profit margins. Berkshire Hathaway Inc harnessed the stock-picking genius of veteran investor Warren Buffett in the United States. And Google Inc created a lightning-fast search engine.

Now PetroChina Co is doing it the Chinese way－with help from government intervention.

Shares of the State-run oil producer have surged 31 percent since June 26, bucking a 13 percent drop in the benchmark Shanghai Composite Index. State-directed funds have piled into shares of China's biggest companies to prop up the country's stock market.

The rally has propelled PetroChina's market capitalization to $382 billion, surpassing Google to become the world's second-largest company behind Apple.

If the widening price gap between PetroChina's mainland shares and Hong Kong counterparts is any guide, international investors do not believe the gains are sustainable.

"The stock is not trading on fundamentals," Neil Beveridge, a Hong Kong-based analyst at Sanford C Bernstein Ltd, said. "The valuation of A shares still looks excessive."